The invention relates to an apparatus for conversion of three-phase power to DC power which, after filtering of switching-frequency spectral components, and even in the event of a single-phase failure, has a sinusoidal profile of the input current and the capability to produce an output voltage which is greater than or less than the peak value of the concatenated power supply system voltage, as is described in the precharacterizing clause of patent claim 1, and to methods for controlling this apparatus.
According to the prior art at the moment, three-phase, pulse-controlled rectifiers with little reaction on the power supply system and without potential isolation generally have a step-up controller characteristic, that is to say the adjustable output voltage is limited at the lower end by the amplitude of the concatenated power supply system voltage. Direct use of such systems, for example for supplying a pulse-controlled inverter system with a variable intermediate-circuit voltage, is thus generally impossible. Furthermore, for systems such as these, the output capacitor must be initially charged during the course of a start-up procedure, and there is no capability for current limiting in the event of an output short circuit.
Known circuits based on step-down controllers admittedly have less implementation complexity, while being restricted to unidirectional power conversion, that is to say they have only one power semiconductor, which can be switched off, per phase, but the output voltage is limited in the upward direction by the concatenated power supply system voltage and, in the event of a single-phase failure, there is no capability to continue operation with a sinusoidal input current and a resistive fundamental frequency power supply system response.
AT 404.415 describes a unidirectional pulse-controlled converter circuit with a wide input voltage range and with a wide output voltage range for a given input voltage, whose implementation likewise requires only one active device which can be switched off per phase, and which allows operation with little reaction on the power system even in the event of a phase failure. However, this system is characterized by a high reverse voltage load on the active devices and has two coupling capacitors with a high current load and two magnetic energy stores per phase, so that it is not possible to take account of the general requirement for industrial electronic systems for a low level of complexity in the power section, or to take account of the requirement for a minimum number of passive components.
The object of the invention is thus to provide a three-phase, pulse-controlled rectifier system in which the power system current has a sinusoidal profile and which has an output voltage range which is not restricted by the concatenated power supply system voltage, in two-phase and three-phase operation, and which does not require any apparatus for initial charging of the output capacitor.
According to the invention, this is achieved by the characterizing features of patent claim 1. Further advantageous refinements of the invention can be found in the dependent claims.
The rectifier system according to the invention may be regarded as being formed by an extension, according to the invention, of the basic structure of a unidirectional three-phase rectifier system, which corresponds to the prior art, with a stabilized output current and/or DC link circuit. The power section of a conventional unidirectional pulse-controlled rectifier system with a current output is formed, in the simplest case, by a voltage-stabilizing, input-side star or delta circuit of filter capacitors, by a three-phase bridge circuit whose conductance state can be controlled by arranging a power semiconductor, which can be turned off, in each phase, and by the current-stabilizing output inductance connected downstream from this bridge.
Each bridge arm of the controllable three-phase bridge has an identical structure and is formed by a positive output diode, which is connected on the cathode side to the output inductance and to whose anode the emitter of the electronic control switch (for example a power transistor) which can be switched off and the anode of a positive input diode, which is connected on the cathode side to the phase input terminal, is connected, and by a negative output diode, which is connected on the cathode side to the collector of the control switch and is connected on the anode side to the negative output busbar, and by a negative input diode, which branches off from the input phase terminal and is likewise connected on the cathode side to the collector of the power transistor. The second terminal of the output inductance is connected to the positive output busbar and, furthermore, an output capacitor which defines the output voltage is generally arranged between the positive and the negative output busbar. One control switch may be switched off in order to prevent current from flowing via the relevant bridge arm while, when the control switch is switched on, that bridge arm has identical characteristics to the bridge arm of a conventional diode bridge. The capability to control the conductance state which this provides may be used to reduce the output voltage of the system in comparison to conventional diode rectification, and in order to reduce the power supply system reactions. As more detailed analysis shows, with appropriate symmetrical-phase control, the output current is split between the input phases such that, after filtering of switching-frequency harmonics by means of the low-pass filter which is formed from the input-side capacitors and the internal power supply system inductance, this results in a sinusoidal power supply system current profile which is in phase with the power supply system voltage. However, the sinusoidal form of the current profile can be maintained only theoretically in the event of a single-phase failure, that is to say with the output inductance having very high inductance values, which are financially unacceptable. Furthermore, in this case, the output voltage range is restricted in the upward direction by the rectified mean value of the remaining concatenated power supply system voltage, that is to say, in some circumstances, it is impossible to maintain the output voltage setting for three-phase operation.
The fundamental idea of the invention is now to avoid these disadvantages by low-complexity integration of a step-up controller stage in the converter structure. For this purpose, a step-up controller diode which is oriented in the direction of the output, is connected between the positive output capacitor terminal and the second terminal of the output inductance and, branching off from the anode of this diode, the emitter of a step-up controller transistor is connected to the negative output voltage rail. The output inductance of the conventional converter carries out the function of the step-up controller inductance. If a voltage which is below the mean value of the maximum output voltage of the controllable diode bridge is intended to be formed at the output of the system, the step-up controller transistor remains switched off, and the operation of the system according to the invention then corresponds to that of a conventional pulse-controlled rectifier. A voltage value above the conventionally maximum achievable output voltage value is achieved by fully sinusoidally driving the three-phase bridge, with a corresponding step-up controller transistor duty ratio. The fundamental operation of the step-up controller in this case corresponds completely to that of a DC voltage/DC voltage step-up controller which is arranged between the diode bridge output and the output capacitor, and therefore does not need to be explained in any more detail.
For three-phase operation, the current in the output inductance is advantageously kept at a constant value. In the event of a single-phase failure, the control transistors for the remaining phases are switched on in those intervals in which the remaining concatenated power supply system voltage is below the output voltage (in the vicinity of the zero crossings). In these intervals, the input current is controlled by the step-up controller to be proportional to the concatenated input voltage. In the subsequent sections of a power supply system cycle in which the input voltage exceeds the output voltage of the system (in the vicinity of the voltage maxima), the step-up controller transistor is switched off, and the current in the output inductance is controlled by appropriately driving the control transistors so as to achieve a continuous progression of the power supply system current and of the input current (from which switching-frequency harmonics have been removed by means of filtering) of the three-phase bridge. (As more detailed analysis shows, the current in the output inductance then follows a profile which is proportional to the square of the input current).
The circuit modification according to the invention thus makes it possible, with relatively little additional complexity, to avoid the major disadvantages of a conventional circuit implementation, that is to say a restricted output voltage range and loss of the sinusoidal form for the power supply system current, in the event of a phase failure. It should also be noted that, despite the step-up controller characteristic, the system can be started up directly, that is to say the initial charge for the output capacitor can be provided by continuously increasing the drive level of the three-phase bridge, starting from an output voltage of zero, with the step-up controller transistor switched off.
Patent claim 2 describes a further advantageous refinement of the invention. As already mentioned, the conductance state of the input-side three-phase bridge is controlled by means of three power transistors, with intervals also occurring in which no voltage is formed at the output of the diode bridge. These time intervals are referred to in the following text as freewheeling intervals. The current which is forced to flow by the output inductance is then passed via the positive output diode, the control transistor and the negative output diode of each bridge arm. Particularly when the output voltage is low, that is to say when the time period during which the freewheeling states are switched on is relatively high, the three voltage drops across the active devices in the current path thus result in relatively high conductance losses. According to the invention this can be avoided by arranging an explicit freewheeling diode between the negative output busbar and the input-side end of the output inductance, which freewheeling diode carries the entire current within the freewheeling intervals and, owing to its relatively low forward-voltage drop, has considerably less forward losses than the series circuit of three active devices, for the same current. A further advantage of this circuit extension according to the invention is that, irrespective of the specific switching states of the control transistors, a closed path is now always available for the stabilized output current at the output of the three-phase bridge, thus also reliably avoiding the occurrence of overvoltages resulting from the control transistors switching in a non-overlapping manner (due to delay times in the signal electronics or in the drive stages).
According to patent claim 3, the output current can also be passed continuously via the diode bridge, when using a control method according to the invention, even without an explicit freewheeling diode. In this case, the associated control transistor remains switched on in intervals which are symmetrical about the maxima of the input phase voltages and have an electrical width of 60xc2x0, and the sinusoidal power supply system current profile and the regulation of the output voltage are achieved by pulsing the two other phases. The operation of such control becomes obviously clear, taking account of the fact that, within these intervals, the concatenated voltages which occur with respect to the non-switching phase and the sinusoidal input currents which are to formed on average over one pulse period and which are proportional to the power supply system voltage do not have any change in their mathematical sign, and the system may thus be regarded as a DC voltage/DC voltage step-down controller, which allows sinusoidal modulation of the input current based on a constant output current, and allows the production of a constant output voltage based on a sinusoidally varying unipolar input voltage. With the exception of the retention of the switching state in one phase according to the invention, the control mechanism therefore has no special features, and will not be discussed any further here.
The switching-frequency oscillation (the ripple) of the output current from the diode bridge can be minimized by coordination, according to the invention, of the control of the step-up controller transistor and of the control transistors as claimed in patent claim 4. The current ripple is governed by the positive and negative voltage differences and voltage time integrals which occur across the inductance. If both systems have the same clock frequency and the control process is carried out such that the intervals in which the step-up controller is switched on are symmetrical about the freewheeling intervals of the diode bridge, no voltage occurs across the inductance for the coverage of these switching states, and there is hence no change in the current in the inductance, so that the amplitude of the switching-frequency ripple is relatively low. Furthermore, taking account of the voltage differences which occur for conduction of the step-up controller diode and the switching states of the three-phase bridge forming a bridge output voltage, this clearly illustrates the capability to calculate an optimum phase shift (which leads, for example, to the current ripple having a minimum root mean square value) for the same-frequency control, that is to say in each case covering the freewheeling states, of both subsystems, which is of minor importance since it is linked to a defined voltage step-up ratio and to the relatively high complexity of practical implementation, and will thus not be described in any more detail here.